The present emphasis in the development of information storage systems is the capability to store more and more information into a so-called "desk top" sized computer memory system. Those "desk top" sized memory systems which incorporate magnetically recorded hard disc media, such as that used in Winchester disc drive type memory systems, currently have the capacity to store upwards of 20 megabytes of magnetically recorded information. The problem with such memory systems is that by necessity the hard disc media is permanently mounted into the computer. Since the media is not easily removable, the user is limited to whatever portion of the hard disc is remaining for information storage at the time of use. Accordingly, magnetically recorded hard disc media information storage systems ar not viewed as a potential solution to increasing information storage capacity.
So-called "floppy" disc memory systems wherein flexible discs, each having a diameter of either 5.25 inches or 3.50 inches, are used as the storage media provides easily removable storage media. However, the problem with such storage systems is that the present storage capacity of information magnetically recorded on a single floppy disc used in such a system has not yet reached a level equal to that of the hard disc, i.e. a single floppy disc media can only store approximately 1 to 2 megabytes of magnetically recorded information.
Systems for storing information which can be accessed through optical devices have received significant attention due to their potential capacity to store substantially more data, i.e. on the order of 400 to 800 megabytes of information, than that available in either magnetically recorded hard disc or floppy disc storage systems. One reason for the significantly increased storage capacity of optical storage systems is that the diameter of a beam of focused light to be used to read or write information is typically only 1 micrometer (micron). Consequently the density of information stored on a media is much greater than a typical magnetic recording density. Additionally, the media for use in such optical systems can be of a form similar to that of a so-called floppy disc, that is a media which is easily removable.
Unfortunately, major problems continue to plague the development and commercial acceptance of such optical systems, namely the relative slowness by which information can be retrieved compared to magnetic storage systems, the current size restrictions of so-called "desk top" computers and the inability to write/erase and rewrite information many times on a single piece of media.
Consider the current size restrictions. So-called "desk top" computers have been provided with a number of modular components, particularly including information storage systems, which can be added into the casing of the computer to provide a certain degree of customizing to fit a particular need. Since such components can have any one of a number of sizes, the American National Standards Institute has adopted certain external standard dimensions with regard to such components, which standards are generally referred to as full-height and half-height standards. Since the half-height standard appears to be the most desirable for such modular components, a need exists to develop an optical information storage system which will fit into the half-height standard. The half-height standard for modular components is as follows: height 1.625 inches; width 5.75 inches; and depth 8.00 inches.
Present optical storage systems include those found in video disc or compact disc (CD) players, which are of the read only variety and those which are termed write once read many times (WORM) optical storage systems. Writing information onto a disc with current optical systems typically is achieved by burning the information into the media. Presently available media has not been developed where such burning can be easily erased and rewritten
A hybrid of the optical and magnetic information storage systems, so called magneto-optic information storage systems appear to have the potential to not only resolve the desire for increased storage capacity but also the need to erase optical information and rewrite new optical information. It has been estimated the theoretical upper limit of the storage capacity of such systems can be as high as 300 megabytes per square inch of media. However in practice on a 5.25 inch floppy disc yields of approximately 400 to 800 megabytes can be expected.
In magneto-optic storage, data are recorded and erased on a thin film of magnetic material having properties to be described herein. Similar to magnetic recording, information is stored as a sequence of bits, where the magnetic field of the film at a given point is either north-pole-up (a digital 1) or north-pole-down (a digital zero). A blank disc has all of its magnetic poles pointing north-pole-down. The twist with magneto-optic media is that the magnetic field required to flip one magnetic domain from north-pole-down to north-pole-up, i.e. the coercive field, varies greatly with temperature. At room temperature, the coercive field necessary to achieve a north pole flip is so high that an ordinary magnet is too weak. At approximately 150.degree. C., the coercive force required to flip a domain falls almost to zero and a bit can be recorded using known magnetic recording techniques.
Optical techniques are used in a magneto-optic system to heat selected spots on the media which is passing close to a relatively large electro-magnet. In this way, a point on the media can be heated, lowering the coercive field required to write a bit of information and the magnet, depending on its own north pole orientation, can so record the desired bit. Once the laser beam is turned off, the just heated point on the media cools "freezing" the oriented north pole to the desired orientation. To erase information so recorded, the process need only be reversed, that is the point on the media will be heated by the laser beam and the magnet's north pole orientation will be such to orient media based north poles in a down orientation.
Since it is undesirable to have any contact with the media, relatively large electromagnets were utilized. Since such magnets are relatively slow in changing their own pole orientation according to the electrical signal being provided, poles on the media are oriented by modulating, i.e. turning on and off, the laser while the magnetic field remains relatively constant.
Reading information so recorded on a magneto-optic disc is achieved solely through optical components. A lower power light beam is focused onto the media. The reflected light is read from either above or below the media. Because of phenomena known as the Kerr magneto-optic effect and the Faraday effect light reflected from the media or passing through the media will have a slightly different polarization than the light being focused onto the media. The change in polarization will be either clockwise or counterclockwise depending on the north pole orientation at that point. For a more graphical interpretation of the above described magneto-optic operation, reference is made to Freese, Robert P., "Optical disks become erasable", IEEE Spectrum, Feb., 1988pages 41-45.
The problem with such magneto-optic information storage systems is the ability to overwrite information without having to make two passes over the same point on the media. Until the present invention, the primary method for overwriting information in a magneto-optic system was a two step process. First, a pass was made over the media to erase all information in a given track. Second, another pass was made over the same points to now record the desired information. Since during magneto-optic recording the laser is turned on and off at a high frequency while the media is continuously moving over an electromagnet having a relatively constant magnetic field, the two step process was the primary method of assuring that no unwanted information remained on the disc after the overwrite operation was completed.
Attempts to resolve this problem included the use of two optical heads and associated electromagnets apparently arranged in a lead/lag fashion, so that the lead head can erase in the same media pass wherein information is written. Another attempt to resolve the two pass problem was the proposed use of side by side light beams focused on adjacent tracks. Still another proposal for resolving this problem was to keep laser power constant while modulating the magnetic field. This latter proposal has been rejected in Kobori, Hiromichi et al. "New magneto-optic head with a built-in generator for a bias magnetic field", Applied Optics, Vol. 27, No. 4 (Feb. 15, 1988) pages 698-702. The reason given for the rejection was that to obtain high data bit rates, the magnetic field generator (magnetic head) had to be located in close proximity to the media. Consequently it would be difficult not only to adopt a double sided disc but also to preserve disc removability.
Consequently a need still exists for a magneto-optic storage system which is capable of overwriting previously stored information with only one pass without the problems of size and access times found in other optical storage systems.